PHP Image Zoom - Exponential scale - php

Question

Iam trying to calculate a zoom Effect beetween 12 Images. Every Image is 100% larger then the one before. Its geting near to perfect, but there is only a issue at the transition beetween the images. It isn't fluid zoom beetween each image.
Please see the video: http://youtu.be/dUBbDjewpO0
I think the Exponential expression pow() isnt coorect for some reason.
Here is the PHP script, but i cant find the issue:
<?php
$imageFiles=array(
'1.jpg',
'2.jpg',
'3.jpg',
'4.jpg');
$targetFrameRate=$targetDuration='18';
$imageCount = count($imageFiles);
$totalFrames = ($targetFrameRate*$targetDuration);
$sourceIndex = 0;
$firstIndex = 1;
$lastIndex = $totalFrames; //==total frames
$currentScale = 1;//image scaling for first scale
$deltaScale = ((($imageCount-1)*($scaleFactor-$currentScale))/$totalFrames);
for ($i=$firstIndex; $i<=$lastIndex; $i++) {
// prepare filename
$filename = createImageFilename($i, $imageType);
// determine source..
if ($i == $firstIndex) {
$newSourceIndex = 0;
}
else if ($i == $lastIndex) {
$newSourceIndex = ($imageCount-1);
}
else {
$newSourceIndex = intval(($i*($imageCount-1))/$totalFrames);
}
// create frame..
if ($newSourceIndex != $sourceIndex) {
$sourceIndex = $newSourceIndex;
$currentScale = pow($scaleFactor, $sourceIndex);
$nextScale = pow($scaleFactor, ($sourceIndex+1));
$deltaScale = ((($imageCount-1)*($nextScale-$currentScale))/$totalFrames);
copyImage($imageFiles[$sourceIndex],
sprintf('%s/%s', $outputDir, $filename),
$imageWidth,
$imageHeight,
$imageType);
}
else {
createImage($imageFiles[$sourceIndex],
sprintf('%s/%s', $outputDir, $filename),
($currentScale/pow($scaleFactor, $sourceIndex)),
$imageWidth,
$imageHeight,
$imageType);
}
//DEBUG: buffer some values for optional debug-output
if (isDebugOutputEnabled()) {
$debug_idx[$i] = $filename;
$debug_inf[$i] = sprintf('sourceIndex=%d , scale=%01.2f<br />', $sourceIndex, $currentScale);
}
// advance..
$currentScale += $deltaScale;
}
?>
rendering is well
shell_exec('ffmpeg -f image2 -i /var/www/htdocs/image2/i%d.jpg -s 1280x720 -movflags faststart -b:v 5500k -r 18 output.flv');

Answer 1

The problem comes from the fact that you are adding a delta to your scale instead of multiplying it by a constant amount each frame:
$currentScale += $deltaScale;
An exponential zoom means you increase the zoom by a constant factor (not difference) for a given constant amount of time, so you need to change that line to:
$currentScale *= $deltaScale;
and also calculate $deltaScale differently:
$deltaScale = pow($nextScale / $currentScale, ($imageCount-1) / $totalFrames);
This will compute a fractional power of the scale difference between the images, so that when you multiply it with the $currentScale value $totalFrames / ($imageCount-1) times (the number of frame you render between the current scale and next scale), the result will be an increase by a factor of $nextScale / $currentScale.
Simplification:
Because the zoom is at a constant rate for the whole animation, $deltaScale is constant the whole time, so you can compute it outside the loop like this:
$deltaScale = pow($scaleFactor, ($imageCount-1) / $totalFrames);